1. Field of the Invention
The present invention relates to a wavelength multiplexing light source used in an optical wavelength division multiplexing (WDM) transmission system or the like.
In the existing final optical fiber communication system, a WDM transmission method by which a large capacity of information is put into a single optical fiber for the transmission has become mainstream.
This is achieved by a wavelength multiplexing light source which realizes a large capacity of optical transmission, as shown in FIG. 12, by carrying information on laser lights respectively having different wavelengths .lambda.1, .lambda.2, .lambda.3, . . . etc. and outputted from semiconductor lasers LD1, LD2, LD3, . . . etc. as light sources for modulators MOD1, MOD2, MOD3, . . . etc., and by coupling the laser lights at an optical coupler OS. Accordingly, without forcibly increasing a modulation rate (bit rate: b/s) of each of the light sources (channels) LD1, LD2, LD3, . . . etc., a large capacity of transmission (several hundred gigabits/second) can be realized comparatively easily.
On the other hand, in an advanced informationalized society, there is no limits in a demand for an enlargement of capacity and therefore it is the present situation that makers all over the world have been competing desperately for the developments. In order to realize a further enlargement of capacity by e.g. the WDM method, it is necessary only to increase the number of wavelength (channel) to be transmitted. For this reason, such an idea can be hit that as many wavelengths as possible are transmitted by narrowing a wavelength interval between the light sources.
However, in the existing optical fiber communication system, an Er-doped optical fiber amplifier having an amplification band in a wavelength of 1.55 .mu.m has been put into practice, to which band all optical signals have to be confined. Accordingly, it is required to narrow the wavelength interval of each light source and arrange the same in a high density within the wavelength band of 1.55 .mu.m.
Presently, the wavelength interval standardized by the International Telecommunication Union is 100 GHz (.apprxeq.0.8nm) in frequency, and is scheduled to be narrowed up to 50GHz. This is only 0.03% of the oscillation frequency (1.55 .mu.m .apprxeq.193.55THz) of a semiconductor laser as a light source, requiring an oscillation wavelength (frequency) control with an extremely high stability.
2. Description of the Related Art
To control such an oscillation wavelength, a conventional stabilization has been attempted under a temperature control of a light source and an injection current control. A general arrangement for such a stabilization is shown in FIG. 13. A part of the output light of a semiconductor laser LD is divided by a beam splitter BS, and its oscillation wavelength is monitored by an optical device MD. After the oscillation wavelength is converted into an electrical signal by a photo detector PD and an electrical signal processing is then performed to a deviation from a reference wavelength at a signal processor SP, current generated by a Peltie element PE combined with the semiconductor laser LD is fed back and controlled by a temperature controller TC.
The control accuracy for the oscillation wavelength by such a wavelength controller depends on; (1) the stability of the device which monitors the oscillation wavelength; (2) the accuracy of the electrical signal processing; and (3) the stability of the temperature controller. Furthermore, these controls are required to be executed per each light source. If such a high density wavelength multiplexing art is further advanced in the future, the number of the wavelength controller has to be increased accordingly and the control accuracy has to be improved, a very difficult situation being expected.